This invention pertains to a controllable, high pressure exhaust gas recirculation (EGR) system for engines and reduction of emissions.
EGR is conventionally used as a method to control (reduce) the emissions of oxides of nitrogen (primarily NO2 and NO) from engines. In a conventional EGR, a portion of the exhaust gas from the engine is returned to the engine to become a portion of later cylinder charge. Because the oxygen content of the cylinder charge is reduced and the temperature of the gases during and after combustion is reduced, oxides of nitrogen are reduced.
In reciprocating internal combustion engines, during the exhaust gas stroke not all of the exhaust gas leaves the cylinder. This gas that remains (the residual gas) is mixed with the fresh air charge during the intake process. EGR systems increase and provide control of higher than baseline residual gas concentrations in the cylinder. EGR systems may also control EGR flow temperatures and EGR flow within the cylinder.
EGR systems may be categorized in accordance with the flow path of the exhaust gases therein and in accordance with the nature of the processes the recirculated exhaust gas undergoes in the flow path.
Internal EGR Systems: In this class of EGR systems, the parameters that influence the concentration of residuals in the cylinder are utilized to change the amount of residual gas. Important parameters are exhaust valve timing, intake valve timing, and exhaust backpressure. To the extent that one or more of these parameters is variable and controllable, the resultant system can control EGR. In this class of systems, in essence, the cylinder provides its own EGR.
External EGR Systems: In this class of EGR systems, exhaust gas is routed into the cylinder, and the exhaust gas introduced into a given cylinder is provided from more than just that one cylinder.
All prior art methods and devices used to perform the function of exhaust gas recirculation involve picking up part of the exhaust gas flow downstream of the exhaust valve and transferring it to an introduction point in the intake system of the engine.
A conventional EGR system is depicted in FIG. 1. FIG. 1 is a schematic of the air and exhaust flows through an engine and the routes for exhaust gas recirculation. While FIG. 1 is a schematic of a turbocharged engine for the purposes of illustration, such conventional EGR systems can be used with non-turbocharged engines (naturally aspirated engines), as well as with supercharged engines. In a non-turbocharged engine, the units C and perhaps D and J can be eliminated, and in the supercharged unit, J would be eliminated.
The inlet air and exhaust gas components of the conventional system illustrated in FIG. 1 are labeled A through M, respectively and the junctions between adjacent components are labeled 1 through 12, respectively.
Exhaust gas may be picked up at one of points 9, 10, 11, and 12, and introduced into the engine at one of points 1, 2, 3, 4, and 5. In the external loop, the EGR treatment apparatus N is one that is used to control recirculated exhaust gas temperature, to filter or catalyze the exhaust gas, or for other purposes. The EGR pump apparatus O can be any of the commonly know pumps whose purpose is to increase the pressure of the recirculated exhaust gas to allow the recirculated exhaust gas to be introduced at the desired location. The exhaust gas after-treatment apparatus K treats the exhaust gas to remove gaseous and/or particulate pollutants. The heat exchanger D in the intake charge stream is typical of current practice with boosted engines and is used to lower or otherwise control the temperature of the inlet charge.
The flow of EGR is controlled by means of a valve called an EGR valve which can be placed in any of the dotted lines in FIG. 1. The purpose of the EGR valve is to control the flow of EGR into the engine since too little EGR may not give the emission reduction desired and too much EGR can cause combustion problems which can lead to increased smoke, particulates, and unburned fuel emissions.
External systems have a subclass system in which the EGR is admitted through the intakes valve(s). These External Intake-valve-admitted (EI) systems represent current practice for many engines. The two basic types of EI systems are those in which the pressure is characterized as High (EIH) or Low (EIL). An example of an EIH system would be one in which the EGR pickup is in the high pressure areaxe2x80x94upstream of the turbocharger turbine (at 9 in FIG. 1), and the EGR admission is downstream of the turbocharger compressor (at 3 in FIG. 1). This high pressure circuit is an example of an EIH system. An example of an EIL system is one in which the EGR pickup is downstream of the exhaust after-treatment device (at 11 in FIG. 1), and the EGR inlet is downstream of the air cleaner (at 2 in FIG. 1).
The challenges that EGR systems must meet are good cylinder-to-cylinder distribution and fast transient response. If the same amount of exhaust gas relative to fresh charge is not provided to each cylinder, less than optimum performance will result. If the system cannot respond quickly enough to changes in power required of the engine, then the transient response of the engine will be poorer.
A substantial problem with current practice is the ability of an EGR system to respond quickly to changes. Consider the case of a fuel injection engine in which the fuel charge is controlled electronically. With modern fuel injection systems, the change in fuel injected can be quite rapid, changing from a very low quantity, for example, the ideal fuel injection amount, to the full load injection amount on the next injection event. For a Diesel engine, the fact that fuel changes can be made from cycle to cycle has potential for fast response to load demands, if the changes in the air and EGR charge into the engine can be made as quickly. Unfortunately, for current practice the response of the air/EGR charge is much slower than that of the fuel. In the example given, the transition from low fuel quantity to high fuel quantity will result in unacceptable emissions of smoke and particulate matter because for most systems, as the fuel quantity injected increases beyond a given level, the EGR amount is reduced to provide adequate oxygen in the charge to result in good combustion. In the example given, when the fuel is increased dramatically there is too much exhaust gas in the inlet system and even if the EGR valve is switched to the OFF (no flow) position, it takes some time for the volume between the EGR valve and the inlet valves to attain the proper value of EGR (much lower or even zero) for the example case.
Therefore, current systems have their transient response limited by the response of the air/EGR system which is much slower than it could be, leading to poorer transient response and vehicle driveability. The response is slow because the volume of the EGR system from exhaust valve to intake is too large compared to the displacement of the engine.
For current practice EGR systems, the EGR flow is a function of (1) the pressure difference between the EGR pickup point and the EGR admission point and (2) the EGR valve opening. For current high pressure systems for use with turbocharged engines, the relationship pressure difference is such that EGR is possible only over a narrow engine load range from zero to about 30 percent, since above that range the EGR admission point pressure exceeds the pressure at the EGR pickup, and EGR flow is not possible. This problem is discussed in SAE paper 970542.
The conventional EGR systems are not fast enough to provide the desired control during transients and, accordingly, a faster responding system is needed. In addition, the prior art systems are incapable of recirculating exhaust gas during some engine conditions due to an adverse pressure gradient, leading to the need for a higher pressure system.
Accordingly, it is an object of the present invention to provide an exhaust gas recirculation (EGR) system which responds more quickly to changes in engine operation, i.e. a faster transient response.
Another object of the present invention is to provide an EGR system capable of generating high pressures to enable EGR flow over a wide variety of engine conditions.
A further object of the present invention is to provide such a faster response EGR system which is applicable to engines of any aspiration type, naturally aspirated, turbocharged, or supercharged.
Another object of the present invention is to utilize the engine as a pump for the EGR.
The foregoing objectives are achieved by the present invention. Toward this end, the present invention provides a unique exhaust circulation system which controls the flow of EGR in a manner different from the prior art, with emphasis placed on control of EGR from the exhaust side of the engine.
The exhaust gas recirculation system of the present invention is adaptable to most, if not all, internal combustion engines having a plurality of cylinders defining respective combustion chambers, each with its own intake and exhaust valves. The exhaust recirculation system of the present invention includes at least one exhaust gas manifold for receiving exhaust gas from the plural cylinders through exhaust lines respectively connected to exhaust valves of the cylinders. An exhaust gas flow control valve is associated with each of the cylinders and is located within one of the exhaust lines. The exhaust gas flow control (EFC) valves each receive exhaust gas from the cylinder associated therewith and apportions the received exhaust gas between a first portion fed to the exhaust manifold and a second portion fed to another cylinder through an exhaust gas recirculation passage.
The present invention may use an exhaust manifold, portions of which are smaller in flow diameter than the conventional exhaust manifolds.
The present invention adds passages, valving, and controls to the conventional exhaust system to allow the exhaust to be pumped from one cylinder into another using the engine""s own cylinders as exhaust gas pumps, thus providing a high pressure, fast acting EGR system.
Control of the flow from the pumping cylinder to the receiving cylinder is obtained via the action of the Exhaust Flow Control (EFC) valve. This valve controls the exit of exhaust gases to the exhaust manifold. When the EFC valve blocks the path of the exhaust gases to the exhaust manifold, exhaust gas cannot enter the exhaust manifold and, therefore, is pumped from the pumping cylinder into the receiving cylinder. When the EFC valve does not block the path of the exhaust gas to the exhaust manifold, exhaust gas can pass through to the exhaust manifold. There can be one EFC valve for each cylinder or one for more than one cylinder.
For example, in a 4-cylinder engine there are two intake events and two exhaust events for each crankshaft revolution. This means that if, for example, one EFC valve controls the EGR flow for all four cylinders, then the EFC valve must open and close two times during one engine revolution. In an embodiment with two EFC valves controlling the EGR flow of four cylinders, i.e., two cylinders per EFC valve, each EFC valve opens and closes once during one engine revolution. In an embodiment with four EFC valves, each EFC valve opens and closes once during two engine revolutions.
Any of a number of types of valves can be used for the EFC valve in the present invention, including poppet valves, rotary valves, disc valves, etc.
Admission of the pumped EGR is usually to the cylinder via the intake valves. The admission can also be via the exhaust valves, or via both intake and exhaust valves, but a more complicated valve event control is needed. The flow paths for the recirculated exhaust gas can be fully contained within the engine cylinder head, or some parts of the flow path can be external to the cylinder head, and the flow paths can include EGR flow treatment such as heat exchangers and/or filters.
Since the engine is also a pump for the EGR, the performance of the pumping piston in transferring exhaust gas to the receiving cylinder is important. For effective transfer of exhaust gas, the volume of the flow passage from the pumping piston to the receiving cylinder should be small compared to the displacement of the pumping cylinder. For the most flexible system, the volume of the pumping passage could be selected so that the pressure generated by the pumping cylinder exceeds the highest value of inlet manifold pressure for the engine. In this case, EGR can be used at any operational condition of the engine, if necessary.
This pumped EGR system has a rapid response to commands for changes in EGR rate. The EGR rate is controlled by the timing of the EFC valve(s), and the volume of the transfer passages is small.
For the fastest response to a command to reduce EGR rate, an additional flow control valve is positioned at the entrance to the intake port in the embodiment in which the pumped EGR is admitted to the intake system. These valves may be reed valves or check valves or other valves known to those skilled in the art. In this case, the volume to purge approaches zero, and the response is determined by the response of the main EFC valve(s).